Method of descaling metals with molten alkali metal hydroxide baths and compositionstherefor



United States Patent John A. Henricks, Logansport, Ind., assignor toDevex Corporation, Cleveland, Ohio, a corporation of Ohio No Drawing.Application December 7, 1956 Serial No. 626,844

6 Claims. (Cl. 1486.11)

This invention relates to a process for descaling ferrous metals,particularly stainless steels, and modifying the oxide on their surfacesso that they may be drawn and otherwise worked.

This application is -a continuation-in-part of my copending application,Serial No. 274,839, filed March 4, 1952, now abandoned, which in turn isa continuation-in-part of my application, Serial No. 193,290, filedOctober 31, 1950, now reissued Patent No. 24,017, and formerly U. S.Patent 2,588,234.

The bulk of the scale collected on the surface of the stainless steelstock from hot rolling or annealing ordinarily can be removed byimmersing it in a fused caustic bath activated by sodium hydride or astrong oxidizing agent such as sodium nitrate. While such baths removescale from the residual surfaces of the metals, they are not welladapted to drawing or forming without further treatment. Whilelubricants may adhere to the residual surfaces left by such baths, theseverity of subsequent drawing operations is necessarily limited unlessadditional surface treatment is provided such as forming an integraloxalate coating on the work. Oxalate coatings, however, are expensiveand difiicult to control.

It is an object of the present invention to provide a bath that candescale efficiently and can also supply an integral coating for allferrous metals comprising an iron sulfide coating.

It is also an object to provide a method for treating scaled red-hotannealed ferrous metals so that they can be descaled and provided withan integral sulfide coating in the same bath.

It is a further object to provide acceleration of the subsequent acidpickling bath by the accelerating action of the iron sulfide formed inthe fused caustic bath.

The present invention comprises a substantially anhydrous caustic bathwhich is activated with preferably about 1% to by weight of lump orsolid sulfur so that the bath will effectively descale ferrous metalobjects and also provide them with an integral iron sulfide coating.This integral coating, being fusible, acts as an excellent lubricant aswell as a superior prime coating for subsequent application of extremepressure lubricants.

The preferred percentage by weight of elemental sulfur to be dissolvedin an anhydrous bath of sodium hydroxide or the like is about 1% to 5%by weight of the caustic bath. Benefits of the present invention may beobtained with as little as .5% by weight, and also amounts of elementalsulfur greater than 5% by weight may be used but with little increase indescaling action or sulfide coating action.

The elemental sulfur that reduces the iron oxide scale is predominantlyoxidized to the polythionic sulfide and other higher valence forms sothat the total active sulfur content generally remains below 5% byweight. It is advantageous to have the elemental sulfur formpolythionates rather than sodium sulfides because the sulfides tend tohydrolyze to H 8 which is very toxic. When more "ice than 20% to 25%sulfur is used, hydrogen sulfide may be evolved so that there is dangerof toxicity, or fire and explosion due to increased heat of solution andthe increased reaction between the sulfur and caustic.

The sulfur acts as a reducing agent for the iron oxide scale formed onmetal such as stainless steel. Sulfur thus functions as a good descalingagent and also reacts with sodium hydroxide to formsodium sulfide,sodium thiosulfate and other polythionic sulfides so as to react withiron and form an integral iron sulfide coating. The temperature of thebath may be from about 600 F. to 1200" F. in order to obtain the bestdescaling and sulfurizing action.

The preferred method of handling metals which are particularly difiicultto descale, such as stainless steels, is to dip the stainless steel workin an anhydrous caustic bath so as to cover it with a relatively thicklayer of at least several mils thickness of caustic material. Thereafterthe work is quickly quenched in a water quench tank to provide a cleanand pickled surface by removing any residual scale from the surfacethereof. The steps of immersing in the caustic solution and quenching inwater are then preferably repeated in order to provide a fusible ironsulfide coating on the surface without trapping or retaining abrasiveparticles in the sulfide layer.

The following example illustrates my invention: j

EXAMPLE 1 Fused caustic sulfurize' I prefer to use a novel bath of fusedsodium hydroxide in which from 1% to 5% lump or solid sulfur isdissolved as the activator. This novel bath will both descale andsulfurize. The work attains a temperature of 800 F. to 1000 F. and whenit is removed from the bath it is filmed with a relatively thick layerof fused sodium hydroxide. The work is quickly quenched in water withalmost explosive violence accompanied by a copious evolution of steam.This violent reaction frees the work surface of a large bulk of theadhering layer of scale. The action is partially chemical in'nature bywhich the lower oxides at the scale-to-metal interface are dissolved assodium ferrite or sodium chromate, but it appears to be largelymechanical and brought about by a violent release of steamwhen the fusedsodium hydroxide is quenched in water so that the scale is burst orcracked from the work surface.

After this initial hot caustic dip and quench, the residual scale isremoved in a hydrochloric or salt-sulfuric acid pickle. In order toobtain a scale free and sulfurized surface, the fused caustic dip andquench has to be repeated after the work has been acid pickled so thatno abrasive particles contaminate the sulfurized layer. Since the quenchbath contains sodium sulfide it could even be used as a sulfurizing bathfor red hot annealed stainless steel to combine the quenching andsulfurizing steps into one operation.

A suitable sulfurizing quench bath for use in connection with thecaustic bath of Example 1 can be made from the following formulation:

This is made up from 2 to 6 ounces per gallon of solution. Upon removalfrom the caustic bath, the work is quenched in this bath andsimultaneously provided with a sulfide coating, the thickness of thecoating depending 7 upon the time which it is left in the bath.

The sodium hydroxide of the fused caustic bath of Example 1 may besubstituted for by other alkali metal hydroxides such as potassiumhydroxide, mixtures of sodium hydroxide and potassium hydroxide, andmixtures of the above hydroxides with other corresponding alkali metalcarbonates. Generally by using any one of the above mix tures, it ispossible to obtain a bath with a lower melting point than would beobtained by using one component alone such as sodium hydroxide. Asulfurizing quench bath such as shown in Example 2, which augments thesulfurizing action of the caustic bath and Water rinse, may preferablybe formed from a dry composition comprising about 10 to 20 parts ofsodium hydroxide, to 45 parts by Weight of elemental sulfur, about 5 to15 parts of starch and about 30 to 50 parts by weight of trisodiumphosphate.

The iron sulfide and other sulfur compounds formed by the reaction ofiron and sulfur in the final caustic bath greatly accelerates chemicalsolution of the ferrous metal in subsequent acid pickling operations byacting as a depolarizer. Thus, the time required to pickle stainless andhigh alloy steels can be greatly decreased.

Although several embodiments of the invention have been herein shown anddescribed, it will be understood that in accordance with the provisionsof the patent statutes, numerous modifications of the process shown maybe resorted to without departing from the spirit of the invention.

What I claim is:

1. A method of descaling ferrous metal by holding the scaled metal in abath of fused sodium hydroxide activated by 1% to 5% elemental sulfurand then quenching the work in water and subsequently pickling 013? thescale residues in an acid bath.

2. A method of descaling ferrous metal and coating said metal with ironsulfide comprising the steps of immersing a scaled metal object in asubstantially anhydrous bath of a molten alkali metal hydroxidecontaining about 0.5 to 25% by weight of sulfur and the oxidationproducts thereof so as to coat said metal object with a relatively thickfilm of bath liquid, quenching said metal object rapidly in a tank ofwater, pickling off the scale residues in an acid bath, and thereafterrepeating said immersing and quenching steps to provide an integral ironsulfide coating on said metal object.

3. A method of descaling ferrous metal and coating said metal with ironsulfide comprising the steps of immersing scaled metal in asubstantially anhydrous bath of a molten alkali metal hydroxidecontaining about 1% to 5% by weight of sulfur and the oxidation productsthereof so as to coat said metal with bath liquid, quenching said metalrapidly in a tank of water, pickling off the scale residues in an acidbath, and thereafter repeating said immersing and quenching steps toprovide an integral iron sulfide coating on said metal.

4. A method of descaling ferrous metal objects and coating said metalobjects with iron sulfide comprising the steps of immersing scaled metalobjects in a substantially anhydrous bath of a molten alkali metalhydroxide activated by 1% to 5% by Weight of elemental lump sulfur so asto coat said metal objects with bath liquid, quenching said metalobjects rapidly into a tank of water, and pickling off the scaleresidues in an acid bath.

5. A method of decaling annealed stainless steel and providing anintegral iron sulfide coating thereon comprising the steps of immersingsaid steel in a substantially anhydrous fused alkali metal hydroxidebath containing about 1% to 5% by weight of sulfur and the oxidationproducts thereof and thereafter quenching the steel in a water bath.

6. A method of descaling annealed stainless steel and providing anintegral iron sulfide coating thereon comprising the steps of immersingsaid steel in a substantially anhydrous fused sodium hydroxide bathcontaining 1% to 5% by weight of sulfur and the oxidation productsthereof, and thereafter quenching the steel in an aqueous quench bathcontaining about 2 to 6 ounces per gallon of bath solution of a drycomposition comprising about 10 to 20 parts by Weight of sodiumhydroxide, 15 to 45 parts by weight of sulfur, about 5 to 15 parts ofstarch, and about 30 to 50 parts by weight of trisodium phosphate.

References Cited in the file of this patent UNITED STATES PATENTS 40,904Brandeis Dec. 15, 1863 339,168 Hall et a1 Apr. 6, 1886 1,572,848 Porteret al Feb. 9, 1926 2,067,530 Ihrig Jan. 12, 1937 2,120,276 Grant June14, 1938 2,223,037 Ihrig Nov. 26, 1940 2,299,139 Grafton Oct. 20, 19422,343,569 Neely et al Mar. 7, 1944 2,350,491 Butler et al June 6, 19442,357,342 Montgomery Sept. 5, 1944 2,440,743 Gary May 4, 1948 2,470,062Whitbeck Oct. 8, 1948 2,489,309 Mills et al Nov. 29, 1949 2,530,837Orozco et a1 Nov. 21, 1950 2,530,838 Orozco Nov. 21, 1950 2,588,234Henricks Mar. 4, 1952 2,609,594 Whitbeck Sept. 9,1952 2,664,399 KluenderDec. 29, 1953 2,697,072 Roden Dec. 14, 1954 2,717,221 Christner Sept. 6,1955

1. A METHO OF DESCALING FERROUS METAL BY HOLDING THE SCALED METAL IN A BATH OF FUSED SODIUM HYDROXIDE ACTIVATED BY 1% TO 5% ELEMENTAL SULFUR AND THEN QUENCHING THE WORK IN WATER AND SUBSEQUENTLY PICKLING OFF THE THE SCALE RESIDUES IN AN ACID BATH. 